FIGS. 1-3 illustrate a conventional dissolved wafer process for fabricating a microstructure, such as a micromechanical or microelectromechanical structure (MEMS), on a substrate 10. As shown in FIG. 1, an intricately patterned, heavily doped microstructure 12, which comprises a simplified microelectromechanical resonator as an example, is formed on a substrate 10 comprising silicon, germanium, gallium arsenide, or other microscopically machinable material. Microstructure 12 may be formed using conventional masking, etching, diffusion, ion implantation, and epitaxial growth techniques. In one technique, the top surface 11 of substrate 10 is heavily doped with a selected dopant to a desired depth, as best illustrated in FIG. 2. The shape of structure 12 can then be defined by trenches 14 in substrate 10 using reactive ion etching, wet chemical etching, or other conventional techniques. Trenches 14 penetrate substrate 10 to below the depth of heavy doping and surround the region forming heavily doped structure 12. Selected raised areas 16 of heavily doped structure 12 are formed by reactive ion etching, wet chemical etching, or other conventional etching or patterned layer deposition techniques that may be performed prior or subsequent to the steps used to form structure 12. Raised areas 16 are subsequently bonded to a second substrate 18, as shown in FIG. 2. FIG. 2 is a cross section of substrate 10 taken along the section lines 2--2 of FIG. 1, with the addition of second substrate 18 bonded atop substrate 10 at raised areas 16. Substrate 18 may comprise any material or combination of materials that can be bonded to raised areas 16 and that resist the selective etch used to dissolve substrate 10. The bonding operation can be accomplished using any conventional technique or combination of techniques such as anodic bonding, optical contacting, thermal bonding, pressure bonding, or soldering, for example. After substrate 18 is bonded to raised areas 16, substrate 10 is dissolved in a selective etch, such as hydrazine or ethylenediamine pyrocatechol (EDP) for silicon substrates, for example. The doped regions 12 and 16, which form the resonator, resist dissolving in the selective etch. After first substrate 10 has dissolved down to trenches 14, microstructure 12 is released from first substrate 10 but it remains bonded to second substrate 18 at raised areas 16, as shown in FIG. 3. This allows structure 12 to function as a microelectromechanical resonator. Heavily doped areas 11 of substrate 10 that are not bonded to substrate 18 simply fall away from structure 12 after substrate 10 has dissolved.
Because heavily doped microstructure 12 is bonded to second substrate 18 only at selected areas 16 and is surrounded by a gap between the two substrates, microstructure 12 and the surfaces of substrate 18 to which it is bonded are exposed to the selective etch for the duration of its application to dissolve first substrate 10. Although the heavily doped material of structure 12 and the material of substrate 18 resist the selective etch, they are nevertheless attacked by the etch to some degree. This undesirable effect limits design flexibility of microstructures and leads to poor device yield. Therefore, improved techniques are needed to increase design flexibility and improve the yield of micromechanical and microelectromechanical devices fabricated by dissolved wafer processes.